Utility Model Publication Showa 56-15426 and Utility Model Publication Heisei 6-17006 disclose a plunger pump wherein a pumping chamber is sealed by means of a high pressure side- seal and a low pressure side- seal through which a plunger slidably extends into the pumping chamber and liquid is introduced through a suction port into an annular space defined between the high pressure side- seal and the low pressure side- seal for their lubrication.
In the plunger pump disclosed in Utility Model Publication Heisei 6-17006, the annular space communicates with the suction port only at one circumferential position thereof so that when the plunger pump is installed horizontally for a horizontal reciprocation of the plunger and at certain circumferential position, communication between the annular space and the suction port may be made below a lower half of the annular space. This makes it difficult to vent sufficiently the air from the annular space.
In the plunger pump disclosed in Utility Model Publication Showa 56-15426, there are provided two passages space apart circumferentially through 180.degree. for introducing liquid through the suction port into the annular space. Negative pressure due to the Venturi arrangement is required to pass liquid through the passages into the annular space and thus, this is complex in structure.
In the prior plunger pumps, when they are installed at certain circumferential positions about a center axis thereof, liquid leaking through a low pressure side-seal will be collected in the annular space adjacent the low pressure side- seal without any discharge thereof.
Utility Model Publication Heisei 51-44165 discloses a piston pump comprising a piston with a suction valve therein, liquid being suctioned from the proximal end of the piston through a suction valve in the piston into a pumping chamber at the distal end of the piston. A space is provided adjacent to the proximal end of the piston and connected to a suction pipe and a return pipe at positions spaced circumferentially from each other through 180.degree.. Since the space is positioned on the side of the suction port for introducing liquid into the pumping chamber, it is different in nature from the sealed annular space in the plunger pump.